Light-emitting diodes (LEDs) are good candidates to replace incandescent and other light sources. LEDs have higher power to light conversion efficiencies than incandescent lamps and longer lifetimes. In addition, LEDs operate at relatively low voltages, and hence, are better adapted for use in many battery-powered devices. Furthermore, LEDs are a better approximation to point sources than a fluorescent source, and hence, are better adapted than fluorescent sources for lighting systems in which a point light source that is collimated or focused by an optical system is required.
LED-based light sources often require multiple packaged LEDs to provide light of the desired color and intensity. The light sources require inexpensive packaging that can be easily connected to a printed circuit board or the like that contains the control circuitry for the light source as well as other components. One type of inexpensive package utilizes a lead frame that is encapsulated in a plastic body. The ends of the leads extend from the body and are wrapped around the body to provide contacts on the bottom of the package. The contacts serve both as mounting pads for the LED and electrical contacts for powering the LED.
The packaged part is attached to the printed circuit board by applying a solder paste to a corresponding pair of pads on the printed circuit board and positioning the LED over the pads such that the LED is in contact with the layer of paste. The paste and printed circuit board are then heated to a temperature that converts the solder paste to molten metal that bonds the package to the printed circuit board when the solder cools. In the molten state, the solder can exert a significant force on the small packaged LED due to the surface tension of the liquid solder. If the solder wets the two leads in an unequal manner or the amount of solder on one of the leads is significantly greater than the amount of solder on the other lead, the surface tension forces can cause the packaged part to move from its original position. In some cases, the movement can result in one end of the package being lifted off of the pad. This type of misalignment can render an assembly using the LED defective, resulting in decreased yield and/or increased repair costs.